Number-of-compressors controlling system

ABSTRACT

A number-of-compressors controlling system which has a simple configuration and can produce compressed air by immediately following the load used of the compressed air is provided. The number-of-compressors controlling system has a plurality of compressors ( 2 ), a receiver tank ( 3 ) in which compressed air is supplied from these compressors ( 2 ) and is fed to a compressed air utilization device, a pressure sensor ( 4 ) provided on the receiver tank ( 3 ), and a number-of-compressors controller ( 5 ) which changes the number of compressors being operated based on the detected pressure of the pressure sensor ( 4 ). A number-of-compressors decreasing pressure as a threshold value whether or not the number of compressors being operated is decreased by the number-of-compressors controller ( 5 ) is set to be lower as the number of compressors being operated is larger.

TECHNICAL FIELD

The present invention relates to a number-of-compressors controllingsystem which has a plurality of air compressors and changes the numberof compressors being operated according to the load used of compressedair. This application claims the priority of Japanese Patent ApplicationNo. 2011-160762, filed on Jul. 22, 2011, the contents of which areherein incorporated by reference.

BACKGROUND ART

Conventionally, as disclosed in Patent Document 1, it is proposed that apressure threshold value which increases or decreases the number ofcompressors being operated be changed based on a pressure and its changerate. In the invention described therein, all the compressors are on-offcontrolled (paragraph No. 0029, and so on).

-   Patent Document 1: Japanese Patent Application Laid-Open (JF-A) No.    2007-120497 (claims, paragraphs Nos. 0140-0155, and FIGS. 15 and 16)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, although the pressure change rate is considered, the number ofcompressors being operated is not considered. In addition, a pluralityof compressors are merely on-off controlled. With this, compressed aircannot be produced by immediately following the load used of thecompressed air.

A problem to be solved by the present invention is to provide anumber-of-compressors controlling system which has a simpleconfiguration to consider and control the number of compressors beingoperated, and can produce compressed air by immediately following theload used of the compressed air.

Means for Solving the Problems

The present invention has been made to solve the above problem and afirst aspect of the invention provides a number-of-compressorscontrolling system having a plurality of compressors, a pressure sensorwhich is provided in a position where compressed air is supplied fromthese compressors and is fed to a compressed air utilization device anddetects the pressure of the compressed air, and a number-of-compressorscontroller which changes the number of compressors being operated basedon the detected pressure of the pressure sensor, wherein anumber-of-compressors decreasing pressure as a threshold value whetheror not the number of compressors being operated is decreased by thenumber-of-compressors controller is set to be lower as the number ofcompressors being operated is larger.

According to the first aspect of the invention, number-of-compressorsdecreasing pressure as a threshold value whether or not the number ofcompressors being operated is decreased is set to be lower as the numberof compressors being operated is larger. As the number of compressorsbeing operated is larger, the contribution rate of each of thecompressors for maintaining the detected pressure at the target pressureis lowered to reduce pressure fluctuations, so that thenumber-of-compressors decreasing pressure can be lowered according tothe increase of the number of compressors being operated. In otherwords, although typically, one compressor is stopped when the detectedpressure is equal to or greater than the number-of-compressorsdecreasing pressure and one compressor is started when the detectedpressure is equal to or less than a number-of-compressors increasingpressure, the number-of-compressors decreasing pressure is lowered asthe number of compressors being operated is larger, thereby enabling thepressure fluctuation width to be reduced.

A second aspect of the invention provides the number-of-compressorscontrolling system according to the first aspect, wherein a plurality ofcompressors are all capacity-controlled even when operated at the sametime, wherein in the case that a load rate at stop is 0% and a load rateat full load is 100%, one compressor being operated is stopped when theload rate of each of the compressors being operated is equal to or lessthan a stop load rate calculated by the following equation:

the stop load rate (%)=(the number of compressors being operated−1)/thenumber of compressors being operated×100.

According to the second aspect of the invention, a plurality ofcompressors are all capacity-controlled even when operated at the sametime, and stop control is performed based on the load rate of each ofthe compressors according to the number of compressors being operated ina manner in which one of two compressors being operated is stopped whenthe load rate of each of the compressors is equal to or less than 50%and one of three compressors being operated is stopped when a load rateof each of the compressors is equal to or less than 67%, so that anoptimum operation can be performed by a simple configuration.

A third aspect of the invention, provides the number-of-compressorscontrolling system according to the first or second aspect, wherein eachof the compressors is capacity-controlled so that the pressure on thedischarge side thereof is maintained between a lower limit pressure PLand an upper limit pressure PH, wherein to stop one compressor beingoperated at the stop load rate according to the number of compressorsbeing operated, the number-of-compressors decreasing pressure is set bythe following equation based on the number of compressors beingoperated:

the number-of-compressors decreasing pressure=((the upper limit pressurePH−the lower limit pressure PL)÷the number of compressors beingoperated)+the lower limit pressure PL.

According to the third aspect of the invention, the stop load rate isconverted to the pressure, thereby enabling easy control.

A fourth aspect of the invention provides the number-of-compressorscontrolling system according to the third aspect, wherein the number ofcompressors being operated is decreased after the detected pressure ofthe pressure sensor continues to be equal to or greater than thenumber-of-compressors decreasing pressure for a set time.

According to the fourth aspect of the invention, the number ofcompressors being operated is decreased after the detected pressure ofthe pressure sensor continues to be equal to or greater than thenumber-of-compressors decreasing pressure for a set time, so that thecompressors can be prevented from being stopped excessively one afteranother.

A fifth aspect of the invention provides the number-of-compressorscontrolling system according to the third or fourth aspect, whereincompressed air from each of the compressors is supplied to a sharedreceiver tank and is then fed to one or a plurality of compressed airutilization devices, wherein the pressure sensor is provided on thereceiver tank, wherein a number-of-compressors increasing pressure as athreshold value whether or not the number of compressors being operatedis increased by the number-of-compressors controller is set to bedifferent based on a pressure change rate ΔP of a detected pressure P ofthe pressure sensor, wherein in the case that the absolute value of thepressure change rate ΔP is less than a first set value ΔP1, onecompressor is started when the detected pressure P of the pressuresensor is equal to or less than a second lower limit pressure PL2 as thenumber-of-compressors increasing pressure, wherein in the case that theabsolute value of the pressure change rate ΔP is equal to or greaterthan the first set value ΔP1 and is less than a second set value ΔP2,one compressor is started when the detected pressure P of the pressuresensor is equal to or less than a first lower limit pressure PL1 as thenumber-of-compressors increasing pressure, when the detected pressure Pis still maintained to be equal to or less than the first lower limitpressure PL1, one compressor is started each time a predeterminedcontinuous start prevention time elapses, and when the detected pressureP is equal to or less than the second lower limit pressure PL2 lowerthan the first lower limit pressure PL1, one more compressor is startedwithout waiting for the elapse of the continuous start prevention time,wherein in the case that the absolute value of the pressure change rateΔP is equal to or greater than the second set value ΔP2, one compressoris started when the detected pressure P of the pressure sensor is equalto or less than the upper limit pressure PH as the number-of-compressorsincreasing pressure, when the detected pressure P is still maintained tobe equal to or less than the upper limit pressure PH, one compressor isstarted each time the predetermined continuous start prevention timeelapses, and when the detected pressure P is equal to or less than thefirst lower limit pressure PL1 lower than the upper limit pressure PH,one more compressor is started without waiting for the elapse of thecontinuous start prevention time.

According to the fifth aspect of the invention, one compressor isstarted when the detected pressure of the pressure sensor is equal to orless than the number-of-compressors increasing pressure, and when thedetected pressure is still maintained to be equal to or less than thenumber-of-compressors increasing pressure, one compressor is startedeach time the predetermined continuous start prevention time elapses,but in the region in which the absolute value of the pressure changerate is equal to or greater than the set value, when the detectedpressure of the pressure sensor is equal to or less than an immediatelynumber-of-compressors increasing pressure, one compressor is startedwithout waiting for the elapse of the predetermined time. In this way,when the pressure change rate is large and is equal to or greater thanthe set value so as to be greatly apart from the target pressure range,one compressor is started when the detected pressure of the pressuresensor is equal to or less than the immediately number-of-compressorsincreasing pressure without waiting for the elapse of the predeterminedtime, so that the difference between the amount of discharge of thecompressors and the amount of use of the compressed air utilizationdevice can be corrected immediately.

A sixth aspect of the invention provides the number-of-compressorscontrolling system according to any one of the first to fifth aspects,wherein the number-of-compressors controller changes the number ofcompressors being operated based on the discharge flow rate ofcompressed air from each of the compressors, or the rotating speed, theelectric current used, or the electricity used of each of thecompressors, in addition to the detected pressure of the pressuresensor.

According to the sixth aspect of the invention, the number ofcompressors being operated is corrected based on the discharge flow rateof compressed air from each of the compressors, or the rotating speed,the electric current used, or the electricity used of each of thecompressors, so that the optimum number of compressors can be operated.

Further, a seventh aspect of the invention provides thenumber-of-compressors controlling system according to the sixth aspect,wherein the number-of-compressors controller monitors the electriccurrent used or the electricity used of each of the compressors, whereinthe compressor with less electric current used or electricity used isoperated on a priority basis.

According to the seventh aspect of the invention, the compressor withless electric current used or electricity used is operated on a prioritybasis, so that the operation efficiency can be increased.

Effect of the Invention

According to the present invention, the number of compressors beingoperated is considered and controlled by a simple configuration, andcompressed air can be produced by immediately following the load used ofthe compressed air.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an embodiment of anumber-of-compressors controlling system of the present invention.

FIG. 2 is a diagram showing an example of a number-of-compressorscontrolling method of the number-of-compressors controlling system ofFIG. 1 and shows the discharged pressure of each of compressors beingoperated, the pressure in a receiver tank, and a number-of-compressorsbeing operated increase or decrease chart.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a specific embodiment of the present invention will bedescribed in detail with reference to the drawings.

FIG. 1 is a schematic diagram showing an embodiment of anumber-of-compressors controlling system of the present invention. Anumber-of-compressors controlling system 1 of this embodiment has aplurality of compressors 2, 2, . . . , a receiver tank 3 to whichcompressed air is supplied from these compressors 2, a pressure sensor 4which detects the pressure in the receiver tank 3, and anumber-of-compressors controller 5 which controls each of thecompressors 2 based on the detected pressure of the pressure sensor 4.

Each of the compressors 2 is an electrically operated air compressor,has a main body driven by a motor, and sucks, compresses, and dischargesoutside air. Compressed air from each of the compressors 2 is fed viathe shared receiver tank 3 to one or a plurality of compressed airutilization devices (not shown).

The compressors 2 may have any configuration such as a screw type, aturbo type, or a reciprocating type, but typically have the sameconfiguration. In addition, the compressors 2 typically have the samedischarge capacity.

Each of the compressors 2 of this embodiment can be capacity-controlled.Here, each of the compressors 2 can be mechanically capacity-controlledby itself. Any specific capacity control configuration can be used, butin this embodiment, capacity control is performed by regulating theopening of a capacity regulating valve (not shown) provided on thesuction side of each of the compressors 2.

The capacity regulating valve regulates the opening by itself so as tomaintain the pressure on the discharge side of each of the compressors 2at the desired pressure. In other words, with the increase of thepressure on the discharge side of each of the compressors 2, thecapacity regulating valve decreases the opening to decrease the amountof suction, whereby the compressor 2 decreases the amount of discharge,and with the decrease of the pressure on the discharge side of each ofthe compressors 2, the capacity regulating valve increases the openingto increase the amount of suction, whereby the compressor 2 increasesthe amount of discharge.

More specifically, in FIG. 2, the capacity regulating valve regulatesthe opening so as to maintain the pressure on the discharge side of eachof the compressors 2 between a lower limit pressure PL and an upperlimit pressure PH. In this case, the capacity regulating valve is fullyopened when the pressure on the discharge side of each of thecompressors 2 is equal to or less than the lower limit pressure PL, andis fully closed when the pressure on the discharge side of each of thecompressors 2 is equal to or greater than the upper limit pressure PH.In addition, between the lower limit pressure PL and the upper limitpressure PH, the opening is proportionally decreased from the lowerlimit pressure PL toward the upper limit pressure PH. In this way, thepressure range between the lower limit pressure PL and the upper limitpressure PH is the capacity regulating valve control range. That is,each of the compressors 2 has a linear characteristic in which thedischarged pressure and the discharged flow rate are in inverseproportion in the defined regulation range of PL to PH by the capacityregulating valve. In other words, the discharged pressure and the loadrate of each of the compressors 2 have a linear function. In case thatthe pressure on the discharge side of each of the compressors 2 exceedsa predetermined stop pressure PS, the compressor 2 is forcefullystopped.

In this embodiment, typically, a plurality of compressors 2 are allcapacity-controlled even when operated at the same time. Therefore, eachof the compressors 2 is not required to have a full load lock functionfor holding the compressors 2 in the full load operation.

While the receiver tank 3 is a hollow case in which compressed air issupplied from each of the compressors 2 and is supplied to one or aplurality of compressed air utilization devices. The pressure sensor 4is provided so that the pressure in the receiver tank 3 can be detected.

The number-of-compressors controller 5 is connected to each of thecompressors 2 and the pressure sensor 4, and controls each of thecompressors 2 based on the detected pressure of the pressure sensor 4.In this embodiment, the presence or absence of the operation of each ofthe compressors 2 (that is, the change of the number of compressorsbeing operated) is switched. A specific controlling method is asfollows.

FIG. 2 is a diagram showing an example of a number-of-compressorscontrolling method of the number-of-compressors controlling system 1 ofthis embodiment and shows the discharged pressure of each of thecompressors 2 being operated, the pressure in the receiver tank 3 (thatis, the detected pressure of the pressure sensor 4), and anumber-of-compressors being operated increase or decrease chart.

The number-of-compressors being operated increase or decrease chart isdivided into a start chart for increasing the number of compressorsbeing operated, as shown in a chart form at the center of FIG. 2, and astop chart for decreasing the number of compressors being operated, asshown in a bar graph form on the right side of FIG. 2. The start chartshows how each of the compressors 2 is started, in other words, how thenumber of compressors being operated is increased, based on a pressure Pin the receiver tank 3 and its change rate ΔP. On the other hand, thestop chart shows how each of the compressors 2 is stopped, in otherwords, how the number of compressors being operated is decreased, basedon the pressure P in the receiver tank 3 and the number of compressorsbeing actually operated at present. These controls are performed basedon the detected pressure P of the pressure sensor 4 and the pressurechange rate ΔP calculated in each predetermined cycle. In thisembodiment, the average value of the detected pressures P in thepredetermined number of computation cycles of the CPU of thenumber-of-compressors controller 5 (e.g., 20 cycles) is used, and theaverage value of the pressure change rates ΔP in the nearestpredetermined time (e.g., the nearest 20 seconds) is used.

The pressure change rate ΔP is the changed pressure in eachpredetermined time. When the pressure change rate ΔP is negative, thepressure in the receiver tank 3 tends to be decreased, and when thepressure change rate ΔP is positive, the pressure in the receiver tank 3tends to be increased. When the amount of compressed air used by thecompressed air utilization device is larger than the amount ofcompressed air discharged by the compressor 2, the pressure in thereceiver tank 3 is decreased, and on the contrary, when the amount ofcompressed air discharged by the compressor 2 is larger than the amountof compressed air used by the compressed air utilization device, thepressure in the receiver tank 3 is increased.

The pressure in the receiver tank 3 becomes slightly lower than thedischarged pressure of the compressors 2 due to the pressure loss of thepiping from the compressors 2 to the receiver tank 3. Therefore, asconnected by the slightly inclined dashed lines in FIG. 2, pressures PL1and PL2 in the receiver tank 3 correspond to discharged pressures PL1′and PL2′ of the compressors 2, respectively. In thenumber-of-compressors controlling system 1 of this embodiment, as isapparent from the stop chart, when the pressure in the receiver tank 3is the upper limit pressure PH in the capacity regulating valve controlrange, all the compressors 2 are stopped to allow the air flow rate tobe zero, so that regarding the upper limit pressure PH, the compressordischarged pressure and the receiver tank pressure are the same.

The number-of-compressors controller 5 compares the detected pressure ofthe pressure sensor 4 with a predetermined pressure value to increase ordecrease the number of compressors 2 being operated. In this case, asshown in the start chart, the pressure value which increases the numberof compressors being operated is set to be different based on thepressure change rate ΔP of the detected pressure P of the pressuresensor 4. That is, the number-of-compressors controller 5 starts onecompressor 2 when the detected pressure P of the pressure sensor 4 isequal to or less than a number-of-compressors increasing pressure A, butthe number-of-compressors increasing pressure A as a threshold valuewhether or not the number of compressors being operated is increased isset to be higher stepwise as the pressure change rate ΔP is increasedtoward the negative side.

In increasing the number of compressors being operated, when thedetected pressure P of the pressure sensor 4 is maintained to be equalto or less than the number-of-compressors increasing pressure A, thenumber-of-compressors controller 5 starts one compressor 2 each time apredetermined time (continuous start prevention time) elapses, but inthe region in which the pressure change rate ΔP is equal to or less thanthe set value (−ΔP1) (that is, ΔP≦−ΔP1), when the detected pressure P ofthe pressure sensor 4 is equal to or less than an immediatelynumber-of-compressors increasing pressure B, one more compressor isstarted without waiting for the elapse of the predetermined time. Theimmediately number-of-compressors increasing pressure B should be set tobe higher as the absolute value of the pressure change rate ΔP islarger.

On the other hand, as shown in the stop chart, the pressure value whichdecreases the number of compressors being operated is set to bedifferent based on the number of compressors 2 being actually operatedat present. That is, the number-of-compressors controller 5 stops onecompressor 2 when the detected pressure P of the pressure sensor 4 isequal to or greater than a number-of-compressors decreasing pressure C,but the number-of-compressors decreasing pressure C as a threshold valuewhether or not the number of compressors being operated is decreased isset to be lower stepwise as the number of compressors being operated islarger.

The number-of-compressors decreasing pressure C should be determined inconsideration of the load rate of each of the compressors 2. That is, inthe case that the load rate at stop is 0% and the load rate at full loadis 100%, one compressor being operated is stopped when the load rate ofeach of the compressors 2 being operated is equal to or less than a stopload rate calculated by the following equation:

the stop load rate (%)=(the number of compressors being operated−1)/thenumber of compressors being operated×100.  [Equation 1]

To stop one compressor 2 at the stop load rate according to the numberof compressors being operated, the number-of-compressors controller 5decreases the number of compressors being operated, as needed, based onthe number-of-compressors decreasing pressure C calculated by thefollowing equation based on the number of compressors being operated:

the number-of-compressors decreasing pressure C=the upper limit pressurePH−{(the upper limit pressure PH−the lower limit pressure PL)×the stopload rate (%)/100}.  [Equation 2]

Equation 2 can be rewritten as follows using Equation 1:

the number-of-compressors decreasing pressure C={(the upper limitpressure PH−the lower limit pressure PL)÷the number of compressors beingoperated}+the lower limit pressure PL.  [Equation 3]

In this way, the number-of-compressors decreasing pressure C can bedefined according to the number of compressors being operated. Asdescribed above, the upper limit pressure PH and the lower limitpressure PL in Equations 2 and 3 are the compressor discharge pressurewhich defines the capacity regulating valve control range, but in thisembodiment, the actual stop control is performed based on the detectedpressure of the pressure sensor 4 provided on the receiver tank 3, sothat the value corrected in consideration of the pressure loss betweeneach of the compressors 2 and the receiver tank 3 is preferably used.However, as described above, regarding the upper limit pressure PH, thecompressor discharge pressure and the receiver tank pressure are thesame. Therefore, regarding the lower limit pressure PL, the valueconverted to the receiver tank pressure is preferably used.Alternatively, the number-of-compressors decreasing pressure C derivedfrom Equations 2 and 3 is strictly the compressor discharge pressure,which is preferably converted to the receiver tank pressure for control.

Hereinafter, specific control will be described with reference to FIG.2. The second lower limit pressure PL2 is set to be lower than the firstlower limit pressure PL1, and the first lower limit pressure PL1 and thesecond lower limit pressure PL2 are set to be lower than the lower limitvalue PL in the capacity regulating valve control range. In addition,the first set value ΔP1 and the second set value ΔP2 are set inconsideration of the discharge capacity at the time of the full loadoperation for one compressor.

(1) the Increase Control of the Number of Compressors 2 being Operated

(1-1) the Case that the Absolute Value of the Pressure Change Rate ΔP isLess than the First Set Value ΔP1. Specifically, −ΔP1<ΔP<+ΔP1.

One compressor is started when the detected pressure P of the pressuresensor 4 is equal to or less than the second lower limit pressure PL2 asthe number-of-compressors increasing pressure A. Thereby, typically, thepressure is above the second lower limit pressure PL2, but when the loadused of compressed air continues to be increased during this, thepressure can be maintained to be equal to or less than the second lowerlimit pressure PL2. In that case, one compressor 2 is started each timethe predetermined continuous start prevention time elapses. That is,when the detected pressure P of the pressure sensor 4 is stayed in the“start of one compressor” region in FIG. 2, as long as there are thecompressors 2 being stopped, each of the compressors 2 being stopped isstarted each time the continuous start prevention time elapses.

(1-2) the Case that the Absolute Value of the Pressure Change Rate ΔP isEqual to or Greater than the First Set Value ΔP1 and is Less than theSecond Set Value ΔP2. Specifically, −ΔP2<ΔP=−ΔP1.

One compressor is started when the detected pressure P of the pressuresensor 4 is equal to or less than the first lower limit pressure PL1 asthe number-of-compressors increasing pressure A. In this case, when evenwith start of one compressor, the detected pressure is maintained to beequal to or less than the first lower limit pressure PL1, one compressoris started each time the predetermined continuous start prevention timeelapses, and when the detected pressure is equal to or less than thesecond lower limit pressure PL2 as the immediately number-of-compressorsincreasing pressure B, one more compressor is started without waitingfor the elapse of the continuous start prevention time.

That is, in FIG. 2, when the detected pressure P enters the “start ofone compressor” region to start one compressor but is still stayed inthe region, as long as there are the compressors 2 being stopped, eachof the compressors 2 being stopped is started each time the continuousstart prevention time elapses. In addition, during that, when thedetected pressure P enters the “start of one more compressor” region,one more compressor is started without the elapse of the continuousstart prevention time.

(1-3) The Case that the Absolute Value of the Pressure Change Rate \P isEqual to or Greater than the Second Set Value ΔP2. Specifically,ΔP≦−ΔP2.

When the pressure in the receiver tank 3 is lowered (when the pressurechange rate ΔP is negative, that is, ΔP≦−ΔP2), one compressor is startedeven when the detected pressure P of the pressure sensor 4 is equal toor less than the upper limit value PH in the capacity regulating valvecontrol range as the number-of-compressors increasing pressure A, thatis, is in the capacity regulating valve control range of PL to PH. Inthis case, when even with start of one compressor, the detected pressureis maintained in the capacity regulating valve control range of PL toPH, one compressor is started each time the predetermined continuousprevention time elapses, and when the detected pressure is equal to orless than the first lower limit pressure PL1 as the immediatelynumber-of-compressors increasing pressure B, one more compressor isstarted without waiting for the elapse of the continuous startprevention time.

That is, in FIG. 2, when the detected pressure P enters the “start ofone compressor” region to start one compressor but is still stayed inthe region, as long as there are the compressors 2 being stopped, eachof the compressors 2 being stopped is started each time the continuousstart prevention time elapses. In addition, during that, when thedetected pressure P enters the “start of one more compressor” region,one more compressor is started without the elapse of the continuousstart prevention time.

(2) The Decrease Control of the Number of Compressors 2 being Operated

An air pressure is monitored by the pressure sensor 4 to decrease thenumber of compressors 2 being operated in consideration of the stop loadrate from Equation 1 in a manner in which one of two compressors beingoperated is stopped when the load rate of each of the compressors isequal to or less than 50%, one of three compressors being operated isstopped when the load rate of each of the compressors is equal to orless than 67%, and one of four compressors being operated is stoppedwhen the load rate of each of the compressors is equal to or less than75%.

Thereby, as the number of compressors is increased, a high loadoperation is performed in a manner in which when only one compressor isoperated, the load rate of the compressor is 0 to 100%, when twocompressors are operated, the load rate of each of the compressors is 50to 100%, and when three compressors are operated, the load rate of eachof the compressors is 67 to 100%.

To perform control based on pressure, one compressor should be stoppedwhen the detected pressure P is equal to or greater than thenumber-of-compressors decreasing pressure C according to the number ofcompressors being operated calculated by Equation 3 (or Equation 2). Forinstance, at the time of the operation of two compressors, when thedetected pressure P is equal to or greater than “{(the upper limitpressure PH−the lower limit pressure PL)÷2)}+the lower limit pressurePL”, one compressor 2 is stopped. In addition, at the time of theoperation of three compressors, the detected pressure P is equal to orgreater than “{(the upper limit pressure PH−the lower limit pressurePL)÷3}+the lower limit pressure PL”, one compressor 2 is stopped, and insuch a manner, the number-of-compressors decreasing pressure C is setfrom Equation 3 based on the number of compressors being operated.

Here, the number of compressors 2 being operated is preferably decreasedafter the detected pressure P of the pressure sensor 4 continues to beequal to or greater than the number-of-compressors decreasing pressure Cfor a set time. Thereby, the defined time is required from the stop ofone compressor to the stop of the next compressor, so that thecompressors cannot be stopped excessively one after another.

Only by the pressure in the receiver tank 3, the actual load used ofcompressed air in the compressed air utilization device cannot be found.Accordingly, the number-of-compressors controller 5 may correct thenumber of compressors 2 being operated based on the discharge flow rateof compressed air from each of the compressors 2, or the rotating speed,the electric current used, or the electricity used of each of thecompressors 2, in addition to the detected pressure of the pressuresensor 4.

For instance, an airflow meter is provided in the discharge piping fromeach of the compressors 2 to the receiver tank 3, an airflow meter isprovided in the discharge piping from the receiver tank 3 to thecompressed air utilization device, or the rotating speed and theelectric current of each of the compressors 2 are detected, therebygrasping the load rate, that is, the air load, of each of thecompressors 2. Then, the number (capacity) of compressors 2corresponding to the air load is set, so that the excessive compressor 2should be stopped. In particular, as found from the stop chart of FIG.2, the difference in the number-of-compressors decreasing pressure C issmaller as the number of compressors is larger, with the result that thenumber of compressors being optimally operated can be deviated, wherebythe power consumption in each of the compressors 2 may be monitored tocorrect the number-of-compressors decreasing pressure or the number ofcompressors being operated.

Alternatively, in the above embodiment, each of the compressors 2 iscontrolled according to the pressure in the receiver tank 3, but may becontrolled at the air flow rate, as needed. When electric currentmeasurement is performed, the power consumption with respect to the airload can be managed, so that the energy saving effect according to thepresence or absence of the number-of-compressors controlling system 1 ofthe present invention can also be notified.

In addition, the number-of-compressors controller 5 may monitor theelectric current used or the electricity used of each of the compressors2 and operate the compressor 2 with less electric current used orelectricity used on a priority basis. Further, there is a fear that thecompressors 2 being operated cannot have the same discharge capacity dueto the pressure loss in the piping, the error in the capacity regulatingvalves, and the interference between the devices, so that as well as thepower consumption of each of the compressors 2 may be monitored tooperate the compressor with less power consumption on a priority basis,a notification that there is deviation (difference) above the fixedlevel in the power consumption in each of the compressors 2 may beprovided to promote maintenance therefor.

In the operation of a plurality of compressors at the same time, if onlyone compressor is capacity-controlled and the remaining compressors areoperated at full load, the capacity regulation is performed by only onecompressor, so that when the amount of air used is abruptly decreased,the pressure in the receiver tank 3 can be excessively increased.However, according to the configuration of this embodiment, even in theoperation of a plurality of compressors 2, they are allcapacity-controlled, so that even when the amount of air used isabruptly decreased, the capacity control functions of the compressors 2act in parallel, thereby enabling the pressure in the receiver tank 3 tobe prevented from being excessively increased. Even if the amount of airused is lost suddenly and completely, the air pressure cannot beexcessively increased.

In the operation of a plurality of compressors 2 at the same time, ifonly one compressor 2 is capacity-controlled and the remainingcompressors 2 are operated at full load, each of the compressors 2 needsthe full load lock function for holding the compressors 2 in the fullload operation. However, since this function is not provided in each ofthe compressors 2 as standard, each of the compressors 2 is required tobe remodeled. However, according to the system 1 of this embodiment,since each of the compressors 2 does not require the full load lockfunction, each of the compressors 2 is not required to be remodeled.

In addition, when the number-of-compressors decreasing pressure C islowered according to the increase of the number of compressors beingoperated, the load rate of each of the compressors 2 being operated isincreased, as described above, and in other words, each of thecompressors 2 is operated on the low discharge pressure side. Then,typically, the higher efficiency is provided as each of the compressors2 is operated on the lower discharge pressure side, so that theoperation efficiency can be increased by lowering thenumber-of-compressors decreasing pressure C.

The number-of-compressors controlling system 1 of the present inventionis not limited to the configuration of the above embodiment and can bechanged, as needed. For instance, each of the compressors 2 may have anunloader function. In that case, in the above embodiment, the start andstop of each of the compressors 2 should be performed by loading andunloading each of the compressors 2.

In addition, the capacity controlling method of each of the compressors2 is not limited to the adoption of the capacity regulating valveprovided on the suction side of each of the compressors 2, as in theabove embodiment, but other conventionally known configurations can alsobe adopted. Further, without performing capacity control, all thecompressors 2 may be simply on-off controlled or load-unload controlled.

In addition, in the above embodiment, compressed air from each of thecompressors 2 is fed via the receiver tank 3 to the compressed airutilization device and the pressure sensor 4 is provided on the receivertank 3, but the pressure sensor 4 may be provided in the position wherecompressed air is supplied from each of the compressors 2 or in theposition where compressed air is fed to the compressed air utilizationdevice, other than on the receiver tank 3.

Further, in the above embodiment, the compressors have the sameconfiguration and discharge capacity, which may be different, as needed.For instance, when the compressor having a discharge capacity twice thatof the remaining compressors is included, the compressor may becontrolled as two compressors.

EXPLANATION OF REFERENCE NUMERALS

-   1 Number-of-compressors controlling system-   2 Compressor-   3 Receiver tank-   4 Pressure sensor-   5 Number-of-compressors controller-   A Number-of-compressors increasing pressure-   B Immediately number-of-compressors increasing pressure-   C Number-of-compressors decreasing pressure

1. A number-of-compressors controlling system comprising: a plurality ofcompressors; a pressure sensor which is provided in a position wherecompressed air is supplied from these compressors and is fed to acompressed air utilization device and detects the pressure of thecompressed air; and a number-of-compressors controller which changes thenumber of compressors being operated based on the detected pressure ofthe pressure sensor, wherein a number-of-compressors decreasing pressureas a threshold value whether or not the number of compressors beingoperated is decreased by the number-of-compressors controller is set tobe lower as the number of compressors being operated is larger.
 2. Thenumber-of-compressors controlling system according to claim 1, wherein aplurality of compressors are all capacity-controlled even when operatedat the same time, wherein in the case that a load rate at stop is 0% anda load rate at full load is 100%, one compressor being operated isstopped when the load rate of each of the compressors being operated isequal to or less than a stop load rate calculated by the followingequation:the stop load rate (%)=(the number of compressors being operated−1)/thenumber of compressors being operated×100.
 3. The number-of-compressorscontrolling system according to claim 1, wherein each of the compressorsis capacity-controlled so that the pressure on the discharge sidethereof is maintained between a lower limit pressure PL and an upperlimit pressure PH, wherein to stop one compressor being operated at thestop load rate according to the number of compressors being operated,the number-of-compressors decreasing pressure is set by the followingequation based on the number of compressors being operated:the number-of-compressors decreasing pressure={(the upper limit pressurePH−the lower limit pressure PL)÷the number of compressors beingoperated}+the lower limit pressure PL.
 4. The number-of-compressorscontrolling system according to claim 3, wherein the number ofcompressors being operated is decreased after the detected pressure ofthe pressure sensor continues to be equal to or greater than thenumber-of-compressors decreasing pressure for a set time.
 5. Thenumber-of-compressors controlling system according to claim 3, whereincompressed air from each of the compressors is supplied to a sharedreceiver tank and is then fed to one or a plurality of compressed airutilization devices, wherein the pressure sensor is provided on thereceiver tank, wherein a number-of-compressors increasing pressure as athreshold value whether or not the number of compressors being operatedis increased by the number-of-compressors controller is set to bedifferent based on a pressure change rate ΔP of a detected pressure P ofthe pressure sensor, wherein in the case that the absolute value of thepressure change rate ΔP is less than a first set value ΔP1, onecompressor is started when the detected pressure P of the pressuresensor is equal to or less than a second lower limit pressure PL2 as thenumber-of-compressors increasing pressure, wherein in the case that theabsolute value of the pressure change rate ΔP is equal to or greaterthan the first set value ΔP1 and is less than a second set value ΔP2,one compressor is started when the detected pressure P of the pressuresensor is equal to or less than a first lower limit pressure PL1 as thenumber-of-compressors increasing pressure, when the detected pressure Pis still maintained to be equal to or less than the first lower limitpressure PL1, one compressor is started each time a predeterminedcontinuous start prevention time elapses, and when the detected pressureP is equal to or less than the second lower limit pressure PL2 lowerthan the first lower limit pressure PL1, one more compressor is startedwithout waiting for the elapse of the continuous start prevention time,wherein in the case that the absolute value of the pressure change rateΔP is equal to or greater than the second set value ΔP2, one compressoris started when the detected pressure P of the pressure sensor is equalto or less than the upper limit pressure PH as the number-of-compressorsincreasing pressure, when the detected pressure P is still maintained tobe equal to or less than the upper limit pressure PH, one compressor isstarted each time the predetermined continuous start prevention timeelapses, and when the detected pressure P is equal to or less than thefirst lower limit pressure PL1 lower than the upper limit pressure PH,one more compressor is started without waiting for the elapse of thecontinuous start prevention time.
 6. The number-of-compressorscontrolling system according to claim 1, wherein thenumber-of-compressors controller changes the number of compressors beingoperated based on the discharge flow rate of compressed air from each ofthe compressors, or the rotating speed, the electric current used, orthe electricity used of each of the compressors, in addition to thedetected pressure of the pressure sensor.
 7. The number-of-compressorscontrolling system according to claim 6, wherein thenumber-of-compressors controller monitors the electric current used orthe electricity used of each of the compressors, wherein the compressorwith less electric current used or electricity used is operated on apriority basis.
 8. The number-of-compressors controlling systemaccording to claim 2, wherein each of the compressors iscapacity-controlled so that the pressure on the discharge side thereofis maintained between a lower limit pressure PL and an upper limitpressure PH, wherein to stop one compressor being operated at the stopload rate according to the number of compressors being operated, thenumber-of-compressors decreasing pressure is set by the followingequation based on the number of compressors being operated:the number-of-compressors decreasing pressure={(the upper limit pressurePH−the lower limit pressure PL)÷the number of compressors beingoperated}+the lower limit pressure PL.
 9. The number-of-compressorscontrolling system according to claim 4, wherein compressed air fromeach of the compressors is supplied to a shared receiver tank and isthen fed to one or a plurality of compressed air utilization devices,wherein the pressure sensor is provided on the receiver tank, wherein anumber-of-compressors increasing pressure as a threshold value whetheror not the number of compressors being operated is increased by thenumber-of-compressors controller is set to be different based on apressure change rate ΔP of a detected pressure P of the pressure sensor,wherein in the case that the absolute value of the pressure change rateΔP is less than a first set value ΔP1, one compressor is started whenthe detected pressure P of the pressure sensor is equal to or less thana second lower limit pressure PL2 as the number-of-compressorsincreasing pressure, wherein in the case that the absolute value of thepressure change rate ΔP is equal to or greater than the first set valueΔP1 and is less than a second set value ΔP2, one compressor is startedwhen the detected pressure P of the pressure sensor is equal to or lessthan a first lower limit pressure PL1 as the number-of-compressorsincreasing pressure, when the detected pressure P is still maintained tobe equal to or less than the first lower limit pressure PL1, onecompressor is started each time a predetermined continuous startprevention time elapses, and when the detected pressure P is equal to orless than the second lower limit pressure PL2 lower than the first lowerlimit pressure PL1, one more compressor is started without waiting forthe elapse of the continuous start prevention time, wherein in the casethat the absolute value of the pressure change rate ΔP is equal to orgreater than the second set value ΔP2, one compressor is started whenthe detected pressure P of the pressure sensor is equal to or less thanthe upper limit pressure PH as the number-of-compressors increasingpressure, when the detected pressure P is still maintained to be equalto or less than the upper limit pressure PH, one compressor is startedeach time the predetermined continuous start prevention time elapses,and when the detected pressure P is equal to or less than the firstlower limit pressure PL1 lower than the upper limit pressure PH, onemore compressor is started without waiting for the elapse of thecontinuous start prevention time.
 10. The number-of-compressorscontrolling system according to claim 2, wherein thenumber-of-compressors controller changes the number of compressors beingoperated based on the discharge flow rate of compressed air from each ofthe compressors, or the rotating speed, the electric current used, orthe electricity used of each of the compressors, in addition to thedetected pressure of the pressure sensor.
 11. The number-of-compressorscontrolling system according to claim 3, wherein thenumber-of-compressors controller changes the number of compressors beingoperated based on the discharge flow rate of compressed air from each ofthe compressors, or the rotating speed, the electric current used, orthe electricity used of each of the compressors, in addition to thedetected pressure of the pressure sensor.
 12. The number-of-compressorscontrolling system according to claim 4, wherein thenumber-of-compressors controller changes the number of compressors beingoperated based on the discharge flow rate of compressed air from each ofthe compressors, or the rotating speed, the electric current used, orthe electricity used of each of the compressors, in addition to thedetected pressure of the pressure sensor.
 13. The number-of-compressorscontrolling system according to claim 5, wherein thenumber-of-compressors controller changes the number of compressors beingoperated based on the discharge flow rate of compressed air from each ofthe compressors, or the rotating speed, the electric current used, orthe electricity used of each of the compressors, in addition to thedetected pressure of the pressure sensor.
 14. The number-of-compressorscontrolling system according to claim 9, wherein thenumber-of-compressors controller changes the number of compressors beingoperated based on the discharge flow rate of compressed air from each ofthe compressors, or the rotating speed, the electric current used, orthe electricity used of each of the compressors, in addition to thedetected pressure of the pressure sensor.
 15. The number-of-compressorscontrolling system according to claim 10, wherein thenumber-of-compressors controller changes the number of compressors beingoperated based on the discharge flow rate of compressed air from each ofthe compressors, or the rotating speed, the electric current used, orthe electricity used of each of the compressors, in addition to thedetected pressure of the pressure sensor.
 16. The number-of-compressorscontrolling system according to claim 11, wherein thenumber-of-compressors controller changes the number of compressors beingoperated based on the discharge flow rate of compressed air from each ofthe compressors, or the rotating speed, the electric current used, orthe electricity used of each of the compressors, in addition to thedetected pressure of the pressure sensor.
 17. The number-of-compressorscontrolling system according to claim 12, wherein thenumber-of-compressors controller changes the number of compressors beingoperated based on the discharge flow rate of compressed air from each ofthe compressors, or the rotating speed, the electric current used, orthe electricity used of each of the compressors, in addition to thedetected pressure of the pressure sensor.
 18. The number-of-compressorscontrolling system according to claim 13, wherein thenumber-of-compressors controller changes the number of compressors beingoperated based on the discharge flow rate of compressed air from each ofthe compressors, or the rotating speed, the electric current used, orthe electricity used of each of the compressors, in addition to thedetected pressure of the pressure sensor.